OPTICAL PUMPING

Optical pumping is a technique for producing spin alignment in a gas
of suitable atoms. Circularly polarized light is applied at the
frequency
of an electronic transition from the ground state, causing transitions
to an excited state. Each photon absorbed adds one quantum of angular
momentum
in the direction of the axis of the light beam to the system of atoms.
However, atoms in the single Zeeman sublevel of the ground state with
highest
angular momentum projection cannot be excited because there is no
excited
level of higher angular momentum. Thus a surplus of atoms (if there
were
no relaxation process, all atoms) accumulates in this sublevel,
producing a net macroscopic magnetic moment. This condition can be
detected
by the resulting increased transmission of the pumping light.

The technique is comparatively simple, but is of great utility. It
can
be used to measure hyperfine splitting and nuclear magnetic moments in
suitable atoms, and is the basis for an atomic clock and a low-field
magnetometer.

What one normally does is a "double" resonance experiment, applying
an r.f. field at the frequency corresponding to the Zeeman splitting
interval.
This tends to depolarize the polarized ground state and results in
increased
optical absorption, which is monitored by a photodiode.

The sensitivity of the technique results in part because absorption
of one r.f. photon at Zeeman-splitting resonance leads to the
absorption
of one optical photon, and this amounts to an enormous power gain.

You should be familiar with the principles of optical pumping before
you begin the experiment. For general background, see Bloom (1) and
Carver
(2). Later you may consult Benumof (3) for more detailed theory. We use
a rubidium-85 sample, in an experimental arrangement essentially the
same
as those described in the references. The apparatus was built by
Physics
43 students several years ago.

The light source, filters, 85Rb absorption cell, and
photodiode
are all mounted in a bakelite cylinder. A calibrated solenoid provides
a magnetic field to produce the Zeeman-splitting, while a µ-metal
shield excludes the Earth's field. Before starting, heat the cell
(center
of the bakelite cylinder) to approximately 40 degrees C.

Among the measurements you can make are the following: (You will not
have to do all of (3), (4), and (5).)

Zero field dip: Look for a dip in light transmission when the axial
magnetic
field is swept through H=O. Understand why this happens, on the basis
of
the semi-classical vector model. (Note that the transverse field is
small
but not exactly zero -- µ-metal is not perfect.) What determines
the width of the dip?

Apply a transverse r.f. field and again sweep H, to look for
resonances.
Make a careful study of the effect of r.f. frequency and also r.f.
power
level.

Quadratic Zeeman effect: At higher frequencies (greater than 10 MHz)
the
resonance splits into multiple dips due to higher-order terms (in H) in
the Zeeman effect. From a measurement of this splitting you can
determine
the hyperfine splitting of the electronic ground state of 85Rb
(see Benumof). Use the "red" solenoid to apply a DC bias field slightly
smaller than required for resonance, then sweep with the "black"
solenoid
as before.

Spin relaxation time: The question is how fast (and why) does the
polarization
decay if pumping is stopped (see Benumof).

Measurement of the Earth's field: Remove the apparatus from the
µ-metal
shield and align the axis approximately with the Earth's field. Does
the
resonance measure the component of field along the light beam, or total
field? (Think about how spins precess in a misaligned field.)

After 1 min. turn on DC voltage (B+) between the filaments(-) and the
plates
(+). Do not exceed 180 V (about 17.5 mA). {Varying this will
control
drift; it can be very touchy.}

Connect the HP 8601A signal generator "r.f. output" to the transverse
R.F.
coil and the "aux. Output" to the frequency counter. Set the signal
generator
to CW, 100 KHz to start.

Connect the output of the Photodiode Signal to a DC voltmeter and
oscilloscope
(and later, XY plotter).

Connect the photodetector power as follows:

red = +15V DC

blue = -15V DC

black = ground

Connect black leads of the solenoid (one wire, one plug) to the DC
power
supply/amplifier (HP 467A), in power supply mode. The red leads are not
needed until Part (3) of the experiment.

For automatic sweep of the magnetic field, use the HP 467A in amplifier
mode, and use the ramp generator as its input. For preliminary
observations
at low field on the oscilloscope, use a function generator instead of
the
ramp generator.

If the XY recorder does not have enough zero offset to expand the
scale of the photodiode signal, construct a 1-op.-amp. DC offset
circuit.